1. Field of the Invention
The present invention relates to a loop network applied to a local area network (LAN), and more particularly, to a transmission line duplex system having duplex transmission lines to effect an automatic reconstruction of the network when faults occur in the transmission lines.
2. Description of the Related Art
In a LAN (local area network), a data exchange among a plurality of terminals between computers is made possible by interconnecting a plurality of communication nodes (communication control units) for accommodating terminals and so forth through a loop transmission line.
In general, an advantage of duplex transmission lines is that, even when a fault occurs in one of the transmission lines, another transmission line can be used to continue the data communication and thus a reliable system is obtained.
Also, by forming the duplex transmission lines as two loops for transmitting data in reverse directions, even when both of the two lines are cut, or when a fault occurs in a node, the data communication can be maintained at a minimum scale of the system through a loop back. In this case, the system operation is a degenerated operation.
In addition to the above-mentioned duplex line technology, when the system is recovered, it is necessary to reset the loop back or to expand the loop back to the recovered position of the system.
Based on the above, the present invention provides a system which enables the formation of a loop back and the resetting of each node (including a supervisory node), to obtain a flexible system operation.
As arts related to the present invention, the following publications are known:
(1) U.S. Pat. No. 4,542,496 disclosing a conventional loop back control technology;
(2) Japanese Unexamined Patent Publication (Kokai) No. 57-92,495 disclosing a prior art to the technology disclosed in the above U.S. Pat. No. 4,542,496;
(3) Japanese Unexamined Patent Publication (Kokai) No. 59-40,739 disclosing a loop back technology with a supervisory unit; and
(4) Japanese Unexamined Patent Publication (Kokai) No. 59-57,544 disclosing a loop back control without a supervisory unit. The art disclosed in this document is an alternative to that of the above Japanese Unexamined Patent Publication (Kokai) No. 59-40,739.
In the above-mentioned related art, those directly related to the present invention are the publications (1) and (2).
In these related arts, however, a loop back control by commands from a supervisory unit is complex and requires several steps.